Genesis of the Pb−Zn deposits of the Qingchengzi ore field, eastern Liaoning, China: Constraints from carbonate LA−ICPMS trace element analysis and C–O–S–Pb isotopes

Abstract

The Qingchengzi ore field is an important Pb–Zn polymetallic ore district located in northeastern China, where more than ten Pb–Zn deposits and several Au-Ag deposits occur. The Pb–Zn mineralization is mainly hosted in the marble of the Dashiqiao Formation. Three distinct mineralization styles (strata-bound, open-space-filling and transitional type) are recognized. Two genetic models have been proposed to explain the Pb–Zn mineralization in this district: the sedimentary-exhalative model and the superimposed multi-stage model, in which syn-sedimentary deposition is overprinted by metamorphic and magmatic hydrothermal modification. Based on these three mineralization styles, this study provides new C–O–S–Pb isotopic data and LA–ICPMS trace element analyses of carbonates in order to place more constraints on the genesis of the Pb–Zn deposits.The δ13CPDB values of the syn-ore carbonates range from −5.11‰ to −1.06‰, and their δ18OSMOW values range from 3.30‰ to 13.2‰; these values are distinctively lower than those of barren meta-sedimentary rocks (δ13C=−1.98‰–−0.55‰, δ18O=19.69‰−22.76‰). The depleted C–O isotopic values of syn-ore carbonates are inferred to result from fluid-rock interactions combined with the impact of graphite. In situ LA–ICPMS trace element analyses show that syn-ore carbonates record enriched REE contents, pronounced positive Eu anomalies and lower Y/Ho ratios compared to barren carbonates. The depleted C–O isotopic signatures and distinctive REE characteristics of syn-ore carbonates indicate that the hydrothermal fluid is likely magmatic in origin and has mixed with low δ18O fluids, such as meteoric or formation water. Sulfide minerals have δ34S values ranging from 3.16‰ to 9.14‰ and display a gradually increasing trend from open-space-filling to transitional type to strata-bound mineralization styles, which reflect different mixing proportions of the two end-members of magmatic sulfur and sedimentary sulfur. The Pb isotopic compositions of sulfide minerals are relatively uniform (206Pb/204Pb=17.511–17.883, 207Pb/204Pb=15.549–15.64 and 208Pb/204Pb=37.670–38.178) for different mineralization styles; however, the open-space-filling mineralization records relatively lower lead isotope values. The linear correlation of ore lead isotopes between schist and Triassic intrusions indicates that lead may have originated from a mixture of magmatic and sedimentary end-members. The relatively lower Pb isotopes of the open-space-filling mineralization suggest the presence of higher proportions from a magmatic source. Mineralization styles respond to diverse ore-controlling structures and their proximity to the mineralizing source: open-space-filling mineralization shows the least stratigraphic control, with more pronounced “magmatic marks” (i.e., lower S–Pb isotopic values and more depleted C–O values), compared to strata-bound mineralization.These mineralization styles, combined with C–O–S–Pb isotopic geochemistry, consistently preclude a SEDEX origin for the Pb–Zn deposits in the Qingchengzi district, but suggest that they represent a distal hydrothermal mineralization type related to Triassic magmatic activity.